A printed circuit board or multi-chip module serves as a support substrate for both discrete and embedded electronic components, such as integrated circuits (ICs), resistors, capacitors, inductors, and other components. The printed circuit board or multi-chip module also provides conductive traces for conveying electric current to the terminals of the electronic components. In multi-layer constructs each layer may contain circuitry or embedded components. The layers are electrically connected by means of conductive vertical interconnects (or vias) buried inside the multi-layer structure. Attachment of discrete components is generally accomplished by means of soldering to metallized through-holes or conductive pads or lands.
In recent years, electronic devices have increasingly employed printed circuits made from polymer thick film (PTF) inks. Typically, these films are applied to the substrate by screen-printing. This additive method is environmentally less harmful and much more cost-effective than other known methods of wiring such as, for example, chemical etching of copper foil. The majority of PTF inks employed for the production of electrically conductive traces are thermosetting or thermoplastic resin pastes with silver or copper powder as the conductor. Unfortunately, the PTF ink compositions are not comparable to copper foil wiring in terms of conductivity and adhesion and are at best only poorly solderable. Metal-filled PTF inks also suffer from degradation of electrical conductivity with aging after curing, and they respond poorly to temperature and humidity fluctuations, presumably due to polymer expansion.
In order to be rendered solderable, conductive polymer thick films as currently employed in the art need to be plated with a solderable metal surface. Plating is typically carried out by electroless or electrolytic metal plating. These techniques are well known to those skilled in the art. Plated PTF inks have been used to create conductive and solderable electronic circuit traces as well as plated through-holes for component attachment in printed circuit boards. PTF inks selected for plating typically contain a conductive filler material such as copper, silver or carbon black, along with a thermoplastic, thermosetting or radiation curable organic binder. Plating with solderable metals (e.g., copper or nickel) is achieved with various commercially available electroless and electrolytic plating baths. Often metal plating is preceded by pretreatment steps aimed at exposing the metallic surface of the PTF conductive filler particles. This may include cleaning with organic solvents, mechanical abrasion, acid-etch, and activation with a plating catalyst.
In spite of the advances in the art with respect to the use of PTF materials, improved methods for creating mechanically strong and solderable, conductive traces on printed circuit boards and multi-chip modules would be desirable. By the use of metal-plating, it is possible that a circuit can be soldered to repeatedly and without degradation of the solder-to-circuit-bond. Such improved methods may therefore find use for the attachment of electronic components to electrical circuits or for the general creation of electrical connections. Similarly, through-hole walls may be coated with TLPS paste and plated thereafter.